Polarization multiplexing and transmitting apparatus

ABSTRACT

A polarization multiplexing and transmitting apparatus generates polarization multiplexed light by multiplexing modulated signal components that having varying intensities and are in polarization states orthogonal to each other. The polarization multiplexing and transmitting apparatus includes a converting unit that converts light generated by a light source into signal components having a varying intensity synchronized with a clock signal input thereto and a varying intensity inversely synchronized with the clock signal, respectively; a modulating unit that modulates the signal components, respectively; and a polarization adjusting unit that orthogonalizes polarization states of the signal components.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2007-226940, filed on Aug. 31,2007, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a polarization multiplexing andtransmitting apparatus that generates polarization multiplexed light.

2. Description of the Related Art

In recent years, the realization of high capacity optical communicationsystems has been demanded to cope with the increase in informationtransmitted through a network. As a technique to accomplish such,multiplexing (MUX) that utilizes polarization, a physical quantity oflight, is examined. Polarization multiplexing is performed by using acoupler to couple two light components having polarization statesorthogonal to each other.

FIG. 14 is a conceptual view of light subjected to polarizationmultiplexing. As shown in FIG. 14, axes H and V represent a horizontaldirection and a vertical direction orthogonal to each other.Polarization multiplexed light 1400 includes a horizontal signalcomponent 1410 and a vertical signal component 1420, each having avarying intensity. Here, time-aligned polarization multiplexing wheretimings of the signal component 1410 having a varying intensity and thesignal component 1420 having a varying intensity are in-phase isperformed. A “signal component having a varying intensity” is alsocalled an “intensity-varying signal component”.

FIG. 14 and the following explanation are based on the assumption thatpolarization multiplexed light is obtained by multiplexing components ina linearly polarization state orthogonal to each other. For example, aterm “polarization direction” is used with respect to a polarizationplane of linearly polarized light, and not only linearly polarized lightbut also elliptically polarized light or circularly polarized light maybe actually used provided polarization states are orthogonal, and“polarization direction” should be read as “polarization state” in thiscase.

FIG. 15 is a conceptual view of light subjected to polarizationmultiplexing. As shown in FIG. 15, D. Van Den Borne, et. al., in“1.6-B/S/Hz Spectrally Efficient Transmission Over 1700 Km Of SSMF Using40×85.6-Gb/S Pol. MUX-RZ-DQPSK”, JLT, Vol. 25, No. 1, 2007, describestransmission that is resistant to non-linear noise of an optical fiberand based on time-interleaved polarization multiplexing where signalcomponents 1410 and 1420, each having a varying intensity and includedin the polarization multiplexed light 1400, are staggered by an amountcorresponding to ½ of a pulse repetition cycle in terms of time.

FIG. 16 is a block diagram of a conventional polarization multiplexingand transmitting apparatus. As shown in FIG. 16, a conventionalpolarization multiplexing and transmitting apparatus 1600 disclosed inthe specification of a U.S. patent, U.S. Pat. No. 6,580,535, usespolarization adjusters 1621 and 1622 to orthogonalize polarizationdirections of respective signals having varying intensities output fromoptical senders 1611 and 1612 and also uses an optical adder 1630 to addthese signals, thereby performing polarization multiplexing (see FIG.14).

FIG. 17 is a block diagram of a conventional polarization multiplexingand transmitting apparatus. As shown in FIG. 17, a conventionalpolarization multiplexing and transmitting apparatus 1700 disclosed inthe specification of a U.S. patent, U.S. Pat. No. 5,111,322, uses asplitter 1710 to split an RZ (Return-to-Zero) optical pulse streamoutput from a mode-locked laser 1710 (M.L.L.) into respective streamsand modulators 1731 and 1732 (MOD.) to modulate the respective streams,and couples the modulated respective streams in a multiplexer 1740 (POL.SPLITTER), thereby effecting polarization multiplexing.

Here, a delay adjustment circuit 1750 is provided between the modulator1731 and the multiplexer 1740, and the delay adjustment circuit 1750 isused to delay one of the respective streams for one pulse and therebystaggers the streams with respect to each other, thus executingtime-interleaved polarization multiplexing (see FIG. 15). Here, thedelay adjustment circuit 1750 is formed of plural mirrors that divertthe passing respective streams while reflecting them.

However, as the conventional technology depicted in FIG. 16 provides fortwo optical senders (optical senders 1611 and 1612), two laser diodes(LD) to generate continuous wave light are also required. Therefore,this technology has a problem in that apparatus size and manufacturingcost increase. When continuous wave light generated by the LDs areconverted into signals having varying intensities by an optical divider,the power of each signal having a varying intensity is attenuated tohalf of that of the continuous wave light. Therefore, this technologyhas a problem in that the power consumed to obtain a necessary power ofa polarization multiplexed light doubles.

The conventional technology depicted in FIG. 17 has a problem in thatapparatus size and manufacturing cost increase since the delayadjustment circuit 1750, which includes mechanically movable components,is provided. When modulation of polarization multiplexed light isperformed in a bit rate variable mode to cope with various signalformats (e.g., synchronous digital hierarchy (SDH), optical transportnetwork (OTN), or Ether), a time-lag of the respective streams must beadjusted according to a change in bit rate. When using the delayadjustment circuit 1750 to adjust the time-lag of the respectivestreams, this technology has a problem in that the configuration andcontrol of the delay adjustment circuit 1750 are complicated.

To eliminate problems associated with conventional techniques, it is anobject of the present invention to provide a polarization multiplexingand transmitting apparatus that can perform time-interleavedpolarization must while reducing apparatus size.

SUMMARY OF THE INVENTION

It is an object of the present invention to at least solve the aboveproblems in the conventional technologies.

A polarization multiplexing and transmitting apparatus according to oneaspect of the present invention includes a converting unit that convertslight generated by a light source into signal components having avarying intensity synchronized with a clock signal input thereto and avarying intensity inversely synchronized with the clock signal,respectively; a modulating unit that modulates the signal components,respectively; and a polarization adjusting unit that orthogonalizespolarization states of the signal components.

A polarization multiplexing and transmitting apparatus according toanother aspect of the present invention includes an optical switch thatdistributes light generated by a light source to two paths at a rateaccording to a clock signal input thereto to be convert the light intosignal components having varying intensities that are inverted withrespect to each other; a modulating unit that modulates the signalcomponents; a polarization adjusting unit that orthogonalizespolarization states of the signal components; and a multiplexing unitthat performs polarization multiplexing with respect to the signalcomponents modulated by the modulating unit and having the polarizationstates orthogonalized by the polarization adjusting unit.

A polarization multiplexing and transmitting apparatus according to yetanother aspect of the present invention includes a polarization switchthat, according to an input clock signal, changes a polarization stateof light generated by a light source to a first polarization state and asecond polarization state orthogonal to the first polarization state; apolarization beam splitter that separates the light having thepolarization state changed by the polarization switch, into signalcomponents that are respectively in the first polarization state and thesecond polarization state, and respectively have a varying intensity; amodulating unit that modulates the signal components; and a multiplexingunit that performs polarization multiplexing with respect to the signalcomponents modulated by the modulating unit.

A polarization multiplexing and transmitting apparatus according tostill another aspect of the present invention includes a splitter thatsplits light generated by a light into light components for output totwo paths; a pulsing unit that pulses one of the light components insynchronization with a clock signal input thereto and pulses the otherof the light components in inverse-synchronization with the clocksignal; a modulating unit that modulates signal components havingvarying intensities pulsed by the pulsing unit; a polarization adjustingunit that orthogonalizes polarization states of the signal componentswith respect to each other; and a multiplexing unit that performspolarization multiplexing with respect to the signal componentsmodulated by the modulating unit and having the polarization statesorthogonalized by the polarization adjusting unit.

A polarization multiplexing and transmitting apparatus according tostill another aspect of the present invention includes a polarizationswitch that, according to a clock signal input thereto, changes apolarization state of light generated by a light source to a firstpolarization state and a second polarization state orthogonal to thefirst polarization state; a first modulating unit that modulates asignal component that has a varying intensity, is in the firstpolarization state, and is in the light having the polarization statechanged by the polarization switch; and a second modulating unit that isconnected with the first modulating unit in series and modulates asignal component that has a varying intensity, is in the secondpolarization state, and is in the light having the polarization statechanged by the polarization switch.

A polarization multiplexing and transmitting apparatus according to yetanother aspect of the present invention includes a pulsing unit that, insynchronization with a clock signal input thereto, pulses lightgenerated by a light source; a polarization adjusting unit that adjustsa polarization state of a signal component having a varying intensityand pulsed by the pulsing unit, to a combined state of a firstpolarization state and a second polarization state orthogonal to thefirst polarization state; a first modulating unit that modulates asignal component that has a varying intensity, is in the firstpolarization state, and is in the light that is pulsed by the pulsingunit and has the polarization state adjusted by the polarizationadjusting unit; and a second modulating unit that is connected with thefirst modulating unit in series and modulates a signal component thathas a varying intensity, is in the second polarization state, and is inthe light that is pulsed by the pulsing unit and has the polarizationstate adjusted by the polarization adjusting unit.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram of a polarization multiplexing andtransmitting apparatus according to the present invention;

FIG. 2 is a block diagram of a polarization multiplexing andtransmitting apparatus according to a first embodiment;

FIG. 3 is a time chart for the polarization multiplexing andtransmitting apparatus according to the first embodiment;

FIG. 4 is a block diagram of an example of the polarization multiplexingand transmitting apparatus according to the first embodiment;

FIG. 5 is a block diagram of a polarization multiplexing andtransmitting apparatus according to a second embodiment;

FIG. 6 is a time chart for the polarization multiplexing andtransmitting apparatus according to the second embodiment;

FIG. 7 is a block diagram of a polarization multiplexing andtransmitting apparatus according to a third embodiment;

FIG. 8 is a time chart of the polarization multiplexing and transmittingapparatus according to the third embodiment;

FIG. 9 is a block diagram of a polarization multiplexing andtransmitting apparatus according to a fourth embodiment;

FIG. 10 is a time chart of the polarization multiplexing andtransmitting apparatus according to the fourth embodiment;

FIG. 11 is a block diagram of a polarization multiplexing andtransmitting apparatus according to a fifth embodiment;

FIG. 12 is a view of a time chart of the polarization multiplexing andtransmitting apparatus according to the fifth embodiment;

FIG. 13 is a block diagram of a polarization multiplexing andtransmitting apparatus according to a sixth embodiment;

FIG. 14 is a conceptual view of light subjected to polarizationmultiplexing;

FIG. 15 is a conceptual view of light subjected to polarizationmultiplexing;

FIG. 16 is a block diagram of a conventional polarization multiplexingand transmitting apparatus; and

FIG. 17 is a block diagram of a conventional polarization multiplexingand transmitting apparatus.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the accompanying drawings, exemplary embodiments accordingto the present invention are explained in detail below.

FIG. 1 is a block diagram of a polarization multiplexing andtransmitting apparatus according to the present invention. Thepolarization multiplexing and transmitting apparatus according to thepresent invention generates polarization multiplexed light bymultiplexing respective modulated signal components having varyingintensities and orthogonal polarization directions. As shown in FIG. 1,a polarization multiplexing and transmitting apparatus 100 includes alight source 110, a converter 120, a modulator 130, and a polarizationadjuster 140.

The light source 110 generates and outputs polarized light to theconverter 120. This polarized light may be continuous wave light orpulsed light. An example in which output from the light source 110 iscontinuous wave light will be explained. The light source 110 is, forexample, an LD. The continuous wave light output from the light source110 and a clock signal (CLK) are input to the converter 120. The clocksignal input to the converter 120 is an alternating electrical signalperiodically switching between “0” and “1” and having, for example, asinusoidal wave, a triangular wave, or a rectangular wave.

The converter 120 converts the continuous wave light output from thelight source 110 into an intensity-varying signal component that issynchronized with the input clock signal and an intensity-varying signalcomponent that is inversely synchronized with the clock signal. Theintensity-varying signal component synchronized with the clock signal isan intensity-varying signal component having an excitation timing thatmatches that of the clock signal. The intensity-varying signal componentinversely synchronized with the clock signal is an intensity-varyingsignal component having an excitation timing that matches that of aninverted clock signal.

The intensity-varying signal component synchronized with the clocksignal and the intensity-varying signal component inversely synchronizedwith the clock signal are two signal components having varyingintensities and split to different paths. Alternatively, theintensity-varying signal component synchronized with the clock signaland the intensity-varying signal component inversely synchronized withthe clock signal are two signal components having varying intensitiesand included in light transmitted through one path. The converter 120outputs the respective converted signal components having varyingintensities to the modulator 130.

The modulator 130 receives the respective signal components havingvarying intensities output from the converter 120 and, two data signalsDATA1 and DATA2. The modulator 130 modulates the signal component havinga varying intensity synchronized with the clock signal that is outputfrom the converter 120, based on DATA 1 (first data signal). Themodulator 130 also modulates the signal component having a varyingintensity inversely synchronized with the clock signal and output fromthe converter 120, based on DATA 2 (second data signal).

For example, based on a clock signal synchronized with the clock signalinput to the converter 120 and an inverted clock signal that is theformer clock signal inverted, the modulator 130 synchronizes the timingof modulation and accordingly performs modulation on the respectivesignal components having varying intensities. The modulator 130 outputsthe respective modulated signal components having varying intensities tothe polarization adjuster 140. As a modulation mode of the modulator130, various modulation modes, excluding polarizing modulation, such asintensity modulation, phase modulation, or frequency modulation can beused solely, or a combination thereof may be employed.

The polarization adjuster 140 orthogonalizes polarization directions ofthe respective signal components having varying intensities output fromthe modulator 130 with respect to each other. The polarization adjuster140 is, for example, two polarization maintaining fibers. In this case,one polarization maintaining fiber allows one of the respective signalcomponents having varying intensities to pass therethrough, and outputsthe signal component having a varying intensity allowed to passtherethrough while maintaining a polarization direction of this signalcomponent in a horizontal direction. The other polarization maintainingfiber allows the other signal component having a varying intensity topass therethrough, and outputs the signal component having a varyingintensity allowed to pass therethrough while maintaining a polarizationdirection of this signal component in a vertical direction.

The polarization adjuster 140 may be, for example, two polarizingelements. In this case, one polarizing element allows one of therespective signal components having varying intensities to passtherethrough, and allows a horizontal component alone of the passedsignal component having a varying intensity to pass therethrough and beoutput. The other polarizing element allows the other of the respectivesignal components having varying intensities to pass therethrough, andallows a vertical component alone of the passed signal component havinga varying intensity to pass therethrough and be output.

The polarization adjuster 140 may be a polarization switch that changesa polarization direction to a horizontal direction and a verticaldirection. As the polarization switch, one that applies a voltage to adevice to rapidly rotate input polarized light at a specific angle withreproducibility is known, for example. In this case, a horizontalcomponent and a vertical component of continuous wave light having apolarization direction changed to the horizontal direction and thevertical direction by the polarization adjuster 140 are converted intosignal components having varying intensities by the converter 120.

As a result, the polarization directions of the respective signalcomponents having varying intensities output from the modulator 130become orthogonal to each other, and the respective modulated signalcomponents having varying intensities are output at timings havingopposite signs. Since the respective signal components having varyingintensities are inverted with respect to each other, polarizationmultiplexed light obtained by subjecting the respective signalcomponents having varying intensities to polarization multiplexing istime-interleaved polarization multiplexed light. A configuration inwhich the polarization adjuster 140 is provided downstream from theconverter 120 is explained here; however, the polarization adjuster 140may be provided upstream from the converter 120, or the polarizationadjuster 140 may be integrated with the converter 120.

As explained above, according to the polarization multiplexing andtransmitting apparatus 100, conversion of continuous wave light into twosignal components having varying intensities by the converter 120enables generation of the respective signal components having varyingintensities by using a single LD. Conversion of continuous wave lightinto two signal components that are inverted with respect to each otherand having varying intensities enables staggering of the respectivesignal components having varying intensities with respect to each otherin terms of time without using a delay adjustment circuit. Therefore,time-interleaved polarization multiplexing can be effected whilereducing apparatus size.

FIG. 2 is a block diagram of a polarization multiplexing andtransmitting apparatus according to the first embodiment. As shown inFIG. 2, a polarization multiplexing and transmitting apparatus 200includes alight source 210, an optical switch 220, modulators 231 and232, polarization adjusters 241 and 242, and a polarization beamcombiner 250 (PBC). The light source 210 is equivalent to the lightsource 110 depicted in FIG. 1. The light source 210 generates andoutputs continuous wave light to the optical switch 220 (referencecharacter A).

The optical switch 220 is equivalent to the converter 120 depicted inFIG. 1. The optical switch 220 receives the continuous wave light outputfrom the light source 210 and a clock signal. The optical switch 220distributes the continuous wave light output from the light source 210to two paths at a rate according to the input clock signal. The twopaths include a path connected with the modulator 231 and a pathconnected with the modulator 232, in this example.

Specifically, the optical switch 220 outputs, to the modulator 231, alight component having a power corresponding to the clock signal andincluded in the input continuous wave light (reference character B). Theoptical switch 220 outputs, to the modulator 232, a light component thatis not output to the modulator 231 and included in the input continuouswave light (reference character C). As a result, the continuous wavelight output from the light source 210 is converted into anintensity-varying signal component synchronized with the clock signaland an intensity-varying signal component inversely synchronized withthe clock-signal.

The modulator 231 and the modulator 232 are equivalent to the modulator130 depicted in FIG. 1. The modulator 231 receives DATA1 and the signalcomponent having a varying intensity output from the optical switch 220.The modulator 231 modulates, based on DATA1, the signal component havinga varying intensity output from the optical switch 220. The modulator231 modulates the signal component having a varying intensity insynchronization with the clock signal. The modulator 231 outputs themodulated signal component having the changing intensity to thepolarization adjuster 241.

The modulator 232 receives the signal component having a varyingintensity output from the optical switch 220 and DATA2. The modulator232 modulates the signal component having a varying intensity outputfrom the optical switch 220 based on DATA2. The modulator 232 modulatesthe signal component having a varying intensity to be inverselysynchronized with the clock signal. The modulator 232 outputs, to thepolarization adjuster 242, the modulated signal component having avarying intensity.

The polarization adjusters 241 and 242 are equivalent to thepolarization adjuster 140 depicted in FIG. 1. The polarization adjuster241 and the polarization adjuster 242 adjust polarization directions ofthe respective signal components having varying intensities output fromthe modulator 231 and the modulator 232, thereby orthogonalizing thepolarization directions of the respective signal components havingvarying intensities for input to the polarization beam combiner 250. Alight component having a polarization direction in a horizontaldirection will be referred to as a horizontal component and a lightcomponent having a polarization direction in a vertical direction willbe referred to as a vertical component hereinafter.

The polarization adjusters 241 and 242 output the respectiveintensity-varying signal components having the adjusted polarizationdirections to the polarization beam combiner 250. The polarization beamcombiner 250 combines the signal component having a varying intensityoutput from the polarization adjuster 241 with the signal componenthaving a varying intensity-output from the polarization adjuster 242 toexecute polarization multiplexing with respect to these signalcomponents having varying intensities. The polarization beam combiner250 outputs a polarization multiplexed light obtained by polarizationmultiplexing to an external device (reference character D).

FIG. 3 is a time chart for the polarization multiplexing andtransmitting apparatus according to the first embodiment. In FIG. 3, anabscissa (t) represents a time common to lights or electrical signals atrespective sections in the polarization multiplexing and transmittingapparatus 200 according to the first embodiment. A waveform 310 isindicative of the clock signal (CLK) input to the optical switch 220.H-pol. of an ordinate represents a horizontal component of a light.V-pol. represents a vertical component of a light.

Reference character A denotes waveforms (321 and 322) of the continuouswave light (see reference character A in FIG. 2) output from the lightsource 210 to the optical switch 220. The waveform 321 represents ahorizontal component of the continuous wave light output from the lightsource 210. The waveform 322 represents a vertical component of thecontinuous wave light output from the light source 210. As representedby the waveform 321 and the waveform 322, the continuous wave lightoutput from the light source 210 includes the horizontal componentalone, i.e., does not include the vertical component.

Reference character B designates waveforms (331 and 332) of the signalcomponent having a varying intensity (see reference character B in FIG.2) output from the optical switch 220 to the modulator 231. The waveform331 represents a horizontal component of the signal component having avarying intensity output to the modulator 231. The waveform 332represents a vertical component of the signal component having a varyingintensity output to the modulator 231. As represented by the waveform331, the signal component having a varying intensity output to themodulator 231 is synchronized with the clock signal (see referencenumeral 310).

Reference character C denotes waveforms (341 and 342) of a signalcomponent having a varying intensity (see reference character C in FIG.2) output from the optical switch 220 to the modulator 232. The waveform341 represents a horizontal component of the signal component having avarying intensity output to the modulator 232. The waveform 342represents a vertical component of the signal component having a varyingintensity output to the modulator 241. As represented by the waveform341, the signal component having a varying intensity output to themodulator 232 is inversely synchronized with the clock signal (seereference numeral 310).

A waveform 350 represents a waveform of DATA1 input to the modulator231. As represented by the waveform 350, DATA1 input to the modulator231 is input in synchronization with the clock signal (see referencenumeral 310). A waveform 360 represents a waveform of DATA2 input to themodulator 232. As represented by the waveform 360, DATA2 input to themodulator 232 is input in synchronization with the clock signal (seereference numeral 310).

Reference character D designates waveforms (371 and 372) of thepolarization multiplexed light (see reference character D in FIG. 2)output from the polarization beam combiner 250. The waveform 371represents a horizontal component of the polarization multiplexed lightoutput from the polarization beam combiner 250. The waveform 372represents a vertical component of the polarization multiplexed lightoutput from the polarization beam combiner 250.

As represented by the waveform 371 and the waveform 372, the horizontalcomponent and the vertical component of the polarization multiplexedlight output from the polarization beam combiner 250 become signalcomponents that have a varying intensity and are inverted with respectto each other. Therefore, the polarization multiplexed light output fromthe polarization beam combiner 250 becomes time-interleaved polarizationmultiplexed light having a horizontal component and a vertical componentthat are staggered in terms of time (see FIG. 15).

FIG. 4 is a block diagram of an example of the polarization multiplexingand transmitting apparatus according to the first embodiment. In FIG. 4,like reference numerals denote structures identical to those depicted inFIG. 2, thereby omitting an explanation thereof. As shown in FIG. 4, apolarization multiplexing and transmitting apparatus 400 includes thelight source 210, a clock oscillator 410, an inverting circuit 411, anoptical switch 420, DFF circuits 431 and 432, phase modulators (QPSKmodulators) 441 and 442, polarization maintaining fibers 451 and 452(PMF), and the polarization beam combiner 250.

The light source 210 generates and outputs continuous wave light to theoptical switch 420. The clock oscillator 410 generates a clock signal.The clock oscillator 410 outputs the generated clock signal to theoptical switch 420, the DFF circuit 431, and the DFF circuit 432. Theinverting circuit 411 is provided between the clock oscillator 410 andthe DFF circuit 432, and inverts the clock signal output to the DFFcircuit 432 from the clock oscillator 410, thereby providing an invertedclock signal.

The optical switch 420 is equivalent to the optical switch 220 depictedin FIG. 2. The optical switch 420 is a Mach-Zehnder type optical switchconfigured by providing a Mach-Zehnder type optical waveguide on anelectro-optic substrate. The Mach-Zehnder type optical waveguideincludes a splitter 421, parallel waveguides 422 a and 422 b, and acoupler 423. The splitter 421 splits the continuous wave light outputfrom the light source 210.

The splitter 421 splits and outputs the continuous wave light to theparallel waveguides 422 a and 422 b. The parallel waveguides 422 a and422 b respectively transmit the continuous wave light output from thesplitter 421 to the coupler 423. A phase shifter 424 is provided in theparallel waveguide 422 a. The phase shifter 424 changes a phase of thecontinuous wave light passing through the parallel waveguide 422 a to 0and π according to a clock signal output from the clock oscillator 410.

The coupler 423 couples the continuous wave light respectively outputfrom the parallel waveguides 422 a and 422 b. The coupler 423 includesoutput units 425 a and 425 b. Light that has been coupled by the coupler423 is distributed to the output unit 425 a and the output unit 425 b ata rate according to interference conditions. The coupler 423 is, forexample, an optical coupler having two inputs and two outputs.

For example, when a phase difference of the continuous wave lightrespectively output to the coupler 423 is 0 and results in brightinterference, all components of the light coupled by the coupler 423 areoutput from the output unit 425 a. When the phase difference of thecontinuous wave light respectively output to the coupler 423 is π andresults in dark interference, all components of the light coupled by thecoupler 423 are output from the output unit 425 b.

A phase of the continuous wave light output from the parallel waveguide422 a to the coupler 423 is changed to 0 and π according to the clocksignal. On the other hand, a phase of the continuous wave light outputfrom the parallel waveguide 422 b to the coupler 423 is not changedaccording to the clock signal. Therefore, the phase difference of thecontinuous wave light respectively output to the coupler 423 is changedto 0 and π according to the clock signal.

Hence, the light coupled by the coupler 423 is distributed to the outputunit 425 a and the output unit 425 b at a rate according to the clocksignal. If the phase difference of the continuous wave lightrespectively output thereto becomes 0 when the clock signal is “1” and πwhen the clock signal is “0”, a light component distributed to theoutput unit 425 a becomes an intensity-varying signal component that issynchronized with the clock signal. A light component distributed to theoutput unit 425 b becomes an intensity-varying signal component that isinversely synchronized with the clock signal.

The intensity-varying signal component that is synchronized with theclock signal is output to the phase modulator 441. The intensity-varyingsignal component that is inversely synchronized with the clock signal isoutput to the phase modulator 442. The delay flip flop (DFF) circuit 431receives the clock signal output from the clock oscillator 410 and DATA1(I, Q) that is a binary data signal. The DFF circuit 431 outputs DATA1to the phase modulator 441 in synchronization with the input clocksignal.

The DFF circuit 432 receives an inverted clock signal that is outputfrom the clock oscillator 410 and inverted by the inverting circuit 411,and DATA2 (I, Q) that is a binary data signal. The DFF circuit 432outputs DATA2 to the phase modulator 442 in synchronization with theinput inverted clock signal. That is, the DFF circuit 432 outputs DATA2to the phase modulator 442 in inverse-synchronization with the clocksignal output from the clock oscillator 410.

The phase modulator 441 is equivalent to the modulator 231 depicted inFIG. 2. The phase modulator 441 receives a signal component having avarying intensity output from the optical switch 420 and DATA1 outputfrom the DFF circuit 431. This signal component having a varyingintensity and DATA1 are synchronized with the clock signal. The phasemodulator 441 performs, based on DATA1, quadrature phase shift keyingwith respect to the signal component having a varying intensity andoutputs an obtained result to the polarization maintaining fiber 451.

The phase modulator 442 is equivalent to the modulator 232 depicted inFIG. 2. The phase modulator 442 receives a signal component having avarying intensity output from the optical switch 420 and DATA2 outputfrom the DFF circuit 432. This signal component having a varyingintensity and DATA2 are inversely synchronized with the clock signal.The phase modulator 442 performs, based on DATA2, quadrature phase shiftkeying with respect to the signal component having a varying intensityand outputs a result to the polarization maintaining fiber 452.

The phase modulator 441 that performs quadrature phase shift keying(QPSK) is formed of, for example, two Mach-Zehnder type phase modulatorsthat are connected in parallel and perform 0 and π phase modulations,and a phase shifter that changes a phase of a modulated light from oneof these phase modulators by π/2. The phase modulator 442 can alsoperform quadrature phase shift keying based on the same configuration.

However, configurations of modulators that perform quadrature phaseshift keying other than that explained above are also known, and any ofsuch configuration can be adopted. The same circuit configuration cancope with differential quadrature phase shift keying (DQPSK). When theDFF circuits 431 and 432 are digital/analog (DA) converters, aconfiguration that can cope with quadrature or higher-value phase shiftkeying or quadrature amplitude modulation (QAM) can be provided. Whenoutput amplitudes of the DFF circuits 431 and 432 are insufficient withrespect to necessary driving amplitudes of the phase modulators 441 and442, a non-depicted driving amplification circuit may be provided tocompensate this insufficiency.

The optical switch 420, the phase modulators 441 and 442 may beintegrally provided on one electro-optic substrate. The polarizationmaintaining fibers 451 and 452 (PMF) are equivalent to the polarizationadjusters 241 and 242 depicted in FIG. 2, respectively. The polarizationmaintaining fibers 451 and 452 maintain a state in which polarizationdirections of the respective signal components having varyingintensities output from the phase modulator 441 and the phase modulator442 are orthogonal to each other.

The configuration in which one of the parallel waveguides 422 a and 422b constituting the optical switch 420 includes the phase shifter 424, isexplained in the example depicted in FIG. 4. However, each of therespective parallel waveguides 422 a and 422 b may include the phaseshifter to be driven based on a clock and an inverted clock. Althoughthe configuration in which quadrature phase shift keying is performed,is explained in the example, intensity modulation, phase modulation,frequency modulation, or a combination thereof may be performed, and anintensity modulator or a frequency modulator may be used in place of thephase modulator in such a case.

The polarization maintaining fibers 451 and 452 output the respectivesignal components having varying intensities and maintained polarizationdirections in such a manner that these signal components are combined inan orthogonal polarization state by the polarization beam combiner 250.The polarization beam combiner 250 combines the respective signalcomponents having varying intensities output from the polarizationmaintaining fiber 451 and the polarizing maintaining fiber 452, therebyeffecting polarization multiplexing with respect to these signalcomponents having varying intensities. The polarization beam combiner250 outputs polarized multiplexed light to an external device outside.

As explained above, according to the polarization multiplexing andtransmitting apparatus 200 of the first embodiment, converting thecontinuous wave light into the two signal components having varyingintensities by using the optical switch 220 (optical switch 420) enablesgeneration of the respective signal components having varyingintensities by the single light source 210. Conversion of the continuouswave light into the two inverted signal components having varyingintensities enables the respective signal components having varyingintensities to be staggered with respect to each other in terms of timewithout use of the delay adjustment circuit (see reference numeral 1750in FIG. 17). Therefore, time-interleaved polarization multiplexing canbe executed while reducing apparatus size.

When the continuous wave light is separated into the two inverted signalcomponents having varying intensities, the continuous wave light can beconverted into the two signal components having varying intensitieswithout being attenuated. Therefore, the power consumed to obtain anecessary power for a polarization multiplexed light can be reduced. Forexample, the polarization multiplexing and transmitting apparatus 200can reduce power consumption to approximately ½ as compared with anexample in which a continuous wave light is attenuated by an opticaldivider and converted into signal components having varying intensities.

When a bit rate of the clock signal is variable, polarizationmultiplexed light to be output can have a variable bit rate. Here, sincethe optical switch 220 outputs the respective signal components havingvarying intensities in a state in which these components are invertedwith respect to each other, staggering of the respective signalcomponents having varying intensities in terms of time does not have tobe adjusted according to a change in bit rate. Therefore, simpleconfiguration and control enable time-interleaved polarizationmultiplexing according to a variable bit rate.

FIG. 5 is a block diagram of a polarization multiplexing andtransmitting apparatus according to the second embodiment. As shown inFIG. 5, a polarization multiplexing and transmitting apparatus 500according to the second embodiment includes a light source 510, apolarization switch 520, a polarization beam splitter 530 (PBS), amodulator 541, a modulator 542, and a polarization beam combiner 550.The light source 510 is equivalent to the light source 110 depicted inFIG. 1. The light source 510 generates and outputs continuous wave lightto the polarization switch 520 (reference character A).

The polarization switch 520 and the polarization beam splitter 530 areequivalent to the converter 120 and the polarization adjuster 140depicted in FIG. 1. The polarization switch 520 receives the continuouswave light output from the light source 510 and a clock signal. Thepolarization switch 520 changes a polarization direction of thecontinuous wave light output from the light source 510 to a horizontaldirection (specific direction) and a vertical direction (directionorthogonal to the specific direction) according to the input clocksignal.

The polarization switch 520 outputs the continuous wave light having thechanged polarization direction to the polarization beam splitter 530(reference character B). The polarization beam splitter 530 separatesthe continuous wave light output from the polarization switch 520 into ahorizontal component and a vertical component. The polarization beamsplitter 530 outputs the separated horizontal component to the modulator541 (reference character C), and outputs the separated verticalcomponent to the modulator 542 (reference character D).

The polarization direction of the continuous wave light output from thepolarization switch 520 is changed to the horizontal direction and thevertical direction according to the clock signal. Therefore, respectivelight components output from the polarization beam splitter 530 becomesignal components having varying intensities that are inverted withrespect to each other. Polarization directions of the respective signalcomponents having varying intensities separated by the polarization beamsplitter 530 are orthogonal to each other.

Assuming that the polarization direction of the continuous wave lightbecomes horizontal when the clock signal is “1” and the polarizationdirection of the continuous wave light becomes vertical when the clocksignal is “0”, the signal component having a varying intensity outputfrom the polarization beam splitter 530 to the modulator 541 is anintensity-varying signal component that is synchronized with the clocksignal. The light component output from the polarization beam splitter530 to the modulator 542 is an intensity-varying signal component thatis inversely synchronized with the clock signal.

The modulators 541 and 542 are equivalent to the modulator 130 depictedin FIG. 1. The modulator 541 receives the signal component having avarying intensity output from the polarization beam splitter 530 andDATA1. The modulator 541 modulates the signal component having a varyingintensity output from the polarization beam splitter 530, based onDATA1. The modulator 541 modulates the signal component having a varyingintensity in synchronization with the clock signal. The modulator 541outputs the modulated signal component having a varying intensity to thepolarization beam combiner 550.

The modulator 542 receives the signal component having a varyingintensity output from the polarization beam splitter 530 and DATA2. Themodulator 542 modulates the signal component having a varying intensityoutput from the polarization beam splitter 530, based on DATA2. Themodulator 542 modulates the signal component having a varying intensityin inverse-synchronization with the clock signal. The modulator 542outputs the modulated signal component having a varying intensity to thepolarization beam combiner 550.

Polarization directions of the respective signal components havingvarying intensities input to the polarization beam combiner 550 becomeorthogonal to each other by operations of the polarization switch 520and the polarization beam splitter 530. The polarization beam combiner550 couples the signal component having a varying intensity output fromthe modulator 541 with the signal component having a varying intensityoutput from the modulator 542, thereby performing polarizationmultiplexing with respect to these signal components having varyingintensities. The polarization beam combiner 550 outputs polarizationmultiplexed light to an external device (reference character E).

FIG. 6 is a time chart for the polarization multiplexing andtransmitting apparatus according to the second embodiment. In FIG. 6, anabscissa (t) represents a time common to lights or electrical signals atrespective sections in the polarization multiplexing and transmittingapparatus 500. A waveform 610 is indicative of the clock signal (CLK)input to the polarization switch 520. H-pol. of an ordinate isindicative of a horizontal component of a light. V-pol. is indicative ofa vertical component of a light.

Reference character A denotes waveforms (621 and 622) of the continuouswave light (see reference character A in FIG. 5) output from the lightsource 510 to the polarization switch 520. The waveform 621 represents ahorizontal component of the continuous wave light output from the lightsource 510. The waveform 622 represents a vertical component of thecontinuous wave light output from the light source 510. As representedby the waveform 621 and the waveform 622, the continuous wave lightoutput from the light source 510 includes the horizontal componentalone, i.e., does not include the vertical component.

Reference character B designates waveforms (631 and 632) of thecontinuous wave light (see reference character B in FIG. 5) output fromthe polarization switch 520 to the polarization beam splitter 530. Thewaveform 631 and the waveform 632 represent a horizontal component and avertical component of the continuous wave light output from thepolarization switch 520, respectively. As represented by the waveform631 and the waveform 632, the continuous wave light output from thepolarization switch 520 includes the signal component having a varyingintensity that is synchronized with the clock signal (see referencenumeral 610) and the signal component having a varying intensity that isinversely synchronized with the clock signal.

Reference character C denotes waveforms (641 and 642) of the signalcomponent having a varying intensity (see reference character C in FIG.5) output from the polarization beam splitter 530 to the modulator 541.The waveform 641 represents a horizontal component of the signalcomponent having a varying intensity output to the modulator 541. Thewaveform 642 represents a vertical component of the signal componenthaving a varying intensity output to the modulator 541. As representedby the waveform 641, the signal component having a varying intensityoutput to the modulator 541 is synchronized with the clock signal (seereference numeral 610).

Reference character D designates waveforms (651 and 652) of the signalcomponent having a varying intensity (see reference character D in FIG.5) output from the polarization beam splitter 530 to the modulator 542.The waveform 651 represents a horizontal component of the signalcomponent having a varying intensity output to the modulator 542. Thewaveform 652 represents a vertical component of the signal componenthaving a varying intensity output to the modulator 542. As representedby the waveform 652, the signal component having a varying intensityoutput to the modulator 542 is inversely synchronized with the clocksignal (see reference numeral 610).

A waveform 660 represents a waveform of DATA1 input to the modulator541. As represented by the waveform 660, DATA1 input to the modulator541 is input in synchronization with the clock signal (see referencenumeral 610). A waveform 670 represents a waveform of DATA2 input to themodulator 542. As represented by the waveform 670, DATA2 input to themodulator 542 is input in inverse-synchronization with the clock signal(see reference numeral 610).

Reference character E designates waveforms (681 and 682) of thepolarization multiplexed light (see reference character E in FIG. 5)output from the polarization beam combiner 550. The waveform 681represents a horizontal component of the polarization multiplexed lightoutput from the polarization beam combiner 550. The waveform 682represents a vertical component of the polarization multiplexed lightoutput from the polarization beam combiner 550.

As represented by the waveform 681 and the waveform 682, the horizontalcomponent and the vertical component of the polarization multiplexedlight output from the polarization beam combiner 550 become signalcomponents inverted with respect to each other and having varyingintensities. Therefore, the polarization multiplexed light output fromthe polarization beam combiner 550 becomes time-interleaved polarizationmultiplexed light having a horizontal component and a vertical componentstaggered with respect to each other in terms of time (see FIG. 15).

As explained above, according to the polarization multiplexing andtransmitting apparatus 500 of the second embodiment, use of thepolarization switch 520 and the polarization beam splitter 530 toconvert the continuous wave light into the two signal components havingvarying intensities enables generation of the respective signalcomponents having varying intensities by using the single light source510. When the continuous wave light is converted into two signalcomponents inverted with respect to each other and having varyingintensities, the signal components having varying intensities can bestaggered with respect each other in terms of time without using thedelay adjustment circuit (see reference numeral 1750 in FIG. 17).Therefore, time-interleaved polarization multiplexing can be performedwhile reducing apparatus size.

Separation of the continuous wave light into the two signal componentsinverted with respect to each other and having varying intensitiesenables conversion into the two signal components having varyingintensities without attenuation of the continuous wave light. Therefore,the power consumed to obtain a necessary power for the polarizationmultiplexed light can be reduced. For instance, the polarizationmultiplexing and transmitting apparatus 500 can reduce power consumptionby approximately half as compared with an example in which thecontinuous wave light is attenuated by an optical divider for conversioninto the signal components having varying intensities.

When a bit rate of the clock signal is variable, the polarizationmultiplexed light can have a variable bit rate. Here, since thepolarization beam splitter 530 outputs the respective signal componentshaving varying intensities in a state in which they are always invertedwith respect to each other, temporal staggering of the respective signalcomponents having varying intensities does not have to be adjustedaccording to a change in bit rate. Therefore, time-interleavedpolarization multiplexing according to the variable bit rate can beexecuted based on a simple configuration and control.

The polarization directions of the respective signal components havingvarying intensities output from the polarization beam splitter 530 arealways orthogonal to each other. Therefore, a polarization adjuster thatadjusts the polarization directions of the respective signal componentshaving varying intensities does not have to be provided, thereby furtherreducing apparatus size. The polarization adjuster that adjusts thepolarization directions of the respective signal components havingvarying intensities may be provided to realize a configuration in whichthe polarization directions of the respective signal components havingvarying intensities input to the polarization beam combiner 550 areaccurately orthogonalized.

Although not depicted, providing the polarization adjuster that adjustspolarization states of one or both of the respective signal componentshaving varying intensities output from the polarization beam splitter530 may enable realizing a configuration in which the polarizationdirections of the two signal components having varying intensities areonce matched with each other and then these signal components are inputto the modulators 541 and 542, for example. As this polarizationadjuster, a polarization maintaining fiber or a birefringent plate maybe used, for example. Adoption of such a configuration enables formationof the modulators 541 and 542 on the same substrate having anelectro-optic effect.

FIG. 7 is a block diagram of a polarization multiplexing andtransmitting apparatus according to the third embodiment. As shown inFIG. 7, a polarization multiplexing and transmitting apparatus 700according to the third embodiment includes a light source 710, asplitter 720, optical dividers 731 and 732 (pulse carvers, pulsingunits), modulators 741 and 742, polarization adjusters 751 and 752, anda polarization beam combiner 760. The light source 710 is equivalent tothe light source 110 depicted in FIG. 1.

The light source 710 generates and outputs continuous wave light to thesplitter 720 (reference character A). The splitter 720, the opticaldivider 731, and the optical divider 732 are equivalent to the converter120 depicted in FIG. 1. The splitter 720 splits the continuous wavelight output from the light source 710 for output to the optical divider731 (reference character B) and the optical divider 732 (referencecharacter D).

The optical divider 731 receives the continuous wave light output fromthe splitter 720 and a clock signal. The optical divider 731 is apulsing unit that pulses the continuous wave light output from thesplitter 720 according to the clock signal to become a signal componenthaving a varying intensity synchronized with the clock signal. Theoptical divider 731 outputs the pulsed signal component having a varyingintensity to the modulator 741 (reference character C).

The optical divider 732 receives the continuous wave light output fromthe splitter 720 and an inverted clock signal obtained by inverting theclock signal. The optical divider 732 is a pulsing unit that pulses thecontinuous wave light output from the splitter 720 according to theinverted clock signal to be turned to a signal component having avarying intensity inversely synchronized with the clock signal. Theoptical divider 732 outputs the pulsed signal component having a varyingintensity to the modulator 742 (reference character E).

The modulators 741 and 742 are equivalent to the modulator 130 depictedin FIG. 1. The modulator 741 receives the signal component having avarying intensity output from the optical divider 731 and DATA1. Themodulator 741 modulates the signal component having a varying intensityoutput from the optical divider 731, based on DATA1. The modulator 741also modulates the signal component having a varying intensity insynchronization with the clock signal. The modulator 741 outputs themodulated signal component having a varying intensity to thepolarization adjuster 751.

The modulator 742 receives the signal component having a varyingintensity output from the optical divider 732 and DATA2. The modulator742 modulates the signal component having a varying intensity outputfrom the optical divider 732, based on DATA2. The modulator 742 alsomodulates the signal component having a varying intensity insynchronization with an inverted clock signal obtained by inverting theclock signal. The modulator 742 outputs the modulated signal componenthaving a varying intensity to the polarization adjuster 752.

The polarization adjusters 751 and 752 are equivalent to thepolarization adjuster 140 depicted in FIG. 1. The polarization adjusters751 and 752 adjust polarization directions of the respective signalcomponents having varying intensities output from the modulator 741 and742, respectively, to orthogonalize the polarization directions of thesignal components having varying intensities input to the polarizingbeam combiner 760.

The polarization adjusters 751 and 752 respectively output therespective signal components having varying intensities and adjustedpolarization directions, to the polarization beam combiner 760. Thepolarization beam combiner 760 couples the signal component having avarying intensity output from the polarization adjuster 751 with thesignal component having a varying intensity output from the polarizationadjuster 752 to perform polarization multiplexing with respect to thesesignal components having varying intensities. The polarization beamcombiner 760 outputs polarization multiplexed light obtained bypolarization multiplexing to an external device (reference character F).

FIG. 8 is a time chart of the polarization multiplexing and transmittingapparatus according to the third embodiment. In FIG. 8, an abscissa (t)represents a time common to lights or electrical signals at respectivesections in the polarization multiplexing and transmitting apparatus 700according to the third embodiment. A waveform 810 indicates the clocksignal (CLK) input to the splitter 720. H-pol. of an ordinate indicatesa horizontal component of a light. V-pol. indicates a vertical componentof a light.

Reference character A denotes waveforms (821 and 822) of the continuouswave light (see reference character A in FIG. 7) output from the lightsource 710 to the splitter 720. The waveform 821 represents a horizontalcomponent of the continuous wave light output from the light source 710.The waveform 822 represents a vertical component of the continuous wavelight output from the light source 710. As represented by the waveform821 and the waveform 822, the continuous wave light output from thelight source 710 includes the horizontal component alone, i.e., does notinclude the vertical component.

Reference characters B and C designate waveforms (831 and 832) of thecontinuous wave light (see reference character B in FIG. 7) output fromthe splitter 720 to the optical divider 731 and waveforms (841 and 842)of the signal component having a varying intensity (see referencecharacter C in FIG. 7) output from the optical divider 731 to themodulator 741, respectively. The waveform 831 and the waveform 832represent a horizontal component and a vertical component of thecontinuous wave light output to the optical divider 731.

As represented by the waveform 831, the continuous wave light output tothe optical divider 731 has a power that is ½ of that of the continuouswave light (see reference numeral 821) output from the light source 710.The waveform 841 and the waveform 842 represent a horizontal componentand a vertical component of the signal component having a varyingintensity output from the optical divider 731, respectively. Asrepresented by the waveform 841, the signal component having a varyingintensity output from the optical divider 731 is synchronized with theclock signal (see reference numeral 810).

Reference characters D and E designate waveforms (851 and 852) of thecontinuous wave light (see reference character D in FIG. 7) output fromthe splitter 720 to the optical divider 732 and waveforms (861 and 862)of the signal component having a varying intensity (see referencecharacter E in FIG. 7) output from the optical divider 732 to themodulator 742, respectively. The waveform 851 and the waveform 852represent respectively a horizontal component and a vertical componentof the continuous wave light output to the optical divider 732.

As represented by the waveform 852, the continuous wave light output tothe optical divider 732 has a power that is ½ of that of the continuouswave light (see reference numeral 821) output from the light source 710.The waveform 861 and the waveform 862 represent a horizontal componentand a vertical component of the signal component having a varyingintensity output from the optical divider 732, respectively. Asrepresented by the waveform 862, the signal component having a varyingintensity output from the optical divider 761 is inversely synchronizedwith the clock signal (see reference numeral 810).

The waveform 870 represents a waveform of DATA1 input to the modulator741. As represented by the waveform 870, DATA1 input to the modulator741 is input in synchronization with the clock signal (see referencenumeral 810). The waveform 880 represents a waveform of DATA2 input tothe modulator 742. As represented by the waveform 880, DATA2 input tothe modulator 742 is input in inverse-synchronization with the clocksignal (see reference numeral 810).

Reference numeral F denotes waveforms (891 and 892) of the polarizationmultiplexed light (see reference character F in FIG. 7) output from thepolarization beam combiner 760. The waveform 891 represents a horizontalcomponent of the polarization multiplexed light output from thepolarization beam combiner 760. The waveform 892 represents a verticalcomponent of the polarization multiplexed light output from thepolarization beam combiner 760.

As represented by the waveform 891 and the waveform 892, the horizontalcomponent and the vertical component of the polarization multiplexedlight output from the polarization beam combiner 760 become signalcomponents that are inverted with respect to each other and havingvarying intensities. Therefore, the polarization multiplexed lightoutput from the polarization beam combiner 760 becomes time-interleavedpolarization multiplexed light having a horizontal component and avertical component staggered with respect to each other in terms of time(see FIG. 15).

As explained above, according to the polarization multiplexing andtransmitting apparatus 700 of the third embodiment, use of the splitter720 and, the optical dividers 731 and 732 to convert the continuous wavelight into the two signal components having varying intensities enablesgeneration of the respective signal components having varyingintensities by using the single light source 710. When the continuouswave light is converted into the two signal components that are invertedwith respect to each other and having varying intensities, therespective signal components having varying intensities can be staggeredwith respect each other in terms of time without using the delayadjustment circuit (see reference numeral 1750 in FIG. 17). Therefore,time-interleaved polarization multiplexing can be effected whilereducing apparatus size.

When a bit rate of the clock signal is variable, the polarizationmultiplexed light to be output can have a variable bit rate. Here, sincethe respective signal components having varying intensities are outputfrom the optical dividers 731 and 732 in a state in which they arealways inverted with respect to each other, a temporal staggering of therespective signal components having varying intensities does not have tobe adjusted according to a change in bit rate. Therefore,time-interleaved polarization multiplexing according to a variable bitrate can be performed with a simple configuration and control.

FIG. 9 is a block diagram of a polarization multiplexing andtransmitting apparatus according to the fourth embodiment. As shown inFIG. 9, a polarization multiplexing and transmitting apparatus 900according to the fourth embodiment includes a light source 910, apolarization switch 920, a specific-polarization modulator 930 (firstmodulating unit), and a specific-polarization modulator 940 (secondmodulating unit). The light source 910 is equivalent to the light source110 depicted in FIG. 1. The light source 910 generates and outputscontinuous wave light to the polarization switch 920 (referencecharacter A).

The polarization switch 920 is equivalent to the converter 120 and thepolarization adjuster 140 depicted in FIG. 1. The polarization switch920 receives the continuous wave light output from the light source 910and a clock signal. The polarization switch 920 changes a polarizationdirection of the continuous wave light output from the light source 910to a horizontal direction and a vertical direction according to theinput clock signal. The polarization switch 920 outputs the continuouswave light having the changed polarization direction to thespecific-polarization modulator 930 (reference character B).

The polarization direction of the continuous wave light output from thepolarization switch 920 is changed to the horizontal direction and thevertical direction according to the clock signal. Therefore, thecontinuous wave light output from the polarization switch 920 includes ahorizontal component and a vertical component. Assuming that thepolarization direction of the continuous wave light becomes horizontalwhen the clock signal is “1” and the polarization direction of thecontinuous wave light becomes vertical when the clock signal is “0”, thehorizontal component becomes a signal component having a varyingintensity that is synchronized with the clock signal. The verticalcomponent having the vertical polarization direction becomes a signalcomponent having a varying intensity that is inversely synchronized withthe clock signal.

The specific-polarization modulators 930 and 940 are equivalent to themodulator 130 depicted in FIG. 1. The specific-polarization modulator930 receives the continuous wave light output from the polarizationswitch 920 and DATA1. The specific-polarization modulator 930, based onDATA1, modulates only the signal component having a varying intensityand a horizontal polarization direction in the continuous wave lightoutput from the polarization switch 920. The specific-polarizationmodulator 930, in synchronization with the clock signal, modulates thesignal component having a varying intensity.

The specific-polarization modulator 930 outputs, to thespecific-polarization modulator 940, the continuous wave light includingthe signal component having a varying intensity and horizontalpolarization direction. The specific-polarization modulator 940 receivesthe continuous wave light output from the specific-polarizationmodulator 930 and DATA2. The specific-polarization modulator 940, basedon DATA2, modulates only the signal component having a varying intensityand vertical polarization direction in the continuous wave light outputfrom the specific-polarization modulator 930. The specific-polarizationmodulator 940, in synchronization with an inverted clock signal obtainedby inverting the clock signal, modulates the signal component having avarying intensity.

The specific-polarization modulator 940 outputs the continuous wavelight that includes the modulated signal component having a varyingintensity and a vertical polarization direction, to an external device(reference character C). Specifically, the specific-polarizationmodulator 940 may be formed of, for example, a λ/2 wave plate 941 and aspecific-polarization modulator 942. The λ/2 wave plate 941 changes alinear polarization direction of the continuous wave light output fromthe specific-polarization modulator 930 by 90° for output to thespecific-polarization modulator 942. As a result, the horizontalcomponent and the vertical component included in the continuous wavelight counterchange.

The specific-polarization modulator 942, in synchronization with theinverted clock signal, modulates only the signal component having avarying intensity and horizontal polarization direction in thecontinuous wave light output from the λ/2 wave plate 941. As a result,the specific-polarization modulator 930 can modulate the signalcomponent having a varying intensity and horizontal polarizationdirection in the continuous wave light output from the polarizationswitch 920, and the specific-polarization modulator 940 can modulate thesignal component having a varying intensity and vertical polarizationdirection in the continuous wave light output from the polarizationswitch 920.

A polarization maintaining fiber that changes the polarization directionof the continuous wave light output from the specific-polarizationmodulator 930 by 90° for output to the specific-polarization modulator942 may be provided in place of the λ/2 wave plate 941. Aspecific-polarization modulator that, in synchronization with theinverted clock signal, modulates only the signal component having avarying intensity and vertical polarization direction in the continuouswave light output from the specific-polarization modulator 930 may beprovided in place of the λ/2 wave plate 941 and thespecific-polarization modulator 942.

In a general situation where a polarization state of the continuous wavelight output from the specific-polarization modulator 930 is not linearpolarization, the polarization state of the continuous wave light can beconverted to establish an orthogonal state like the above example byinserting a birefringent element having an appropriate birefringence inplace of the λ/2 wave plate 941.

DATA1 input to the specific-polarization modulator 930 may be split, andthe split DATA1 may be subjected to appropriate amplitude/delayadjustment for input to the specific-polarization modulator 940. Thespecific-polarization modulator 940 compensates DATA2 using DATA1, andexecutes modulation based on the compensated DATA2. DATA2 input to thespecific-polarization modulator 940 may be split, and the split DATA2may be subjected to appropriate amplitude/delay adjustment for input tothe specific-polarization modulator 930. The specific-polarizationmodulator 930 compensates DATA1 using DATA2, and executes modulationbased on the compensated DATA1.

As a result, even if the vertical component is modulated based on DATA1due to an error of the specific-polarization modulator 930, thespecific-polarization modulator 940 compensates DATA2 using DATA1, tomodulate the vertical component, thereby compensating the error.Further, even if the horizontal component is modulated based on DATA2due to an error of the specific-polarization modulator 940, thespecific-polarization modulator 930 compensates DATA2 using DATA1 inadvance to modulate the horizontal component, thereby compensating theerror.

FIG. 10 is a time chart of the polarization multiplexing andtransmitting apparatus according to the fourth embodiment. In FIG. 10,an abscissa (t) represents a time common to lights or electrical signalsat respective sections in the polarization multiplexing and transmittingapparatus 900 according to the fourth embodiment. A waveform 1010represents the clock signal (CLK) input to the polarization switch 920.H-pol. of an ordinate indicates a horizontal component of a light.V-pol. indicates a vertical component of a light.

Reference character A denotes waveforms (1021 and 1022) of thecontinuous wave light (see reference character A in FIG. 9) output fromthe light source 910 to the polarization switch 920. The waveforms 1021and 1022 respectively represent a horizontal and a vertical component ofthe continuous wave light output from the light source 910. Asrepresented by the waveforms 1021 and 1022, the continuous wave lightoutput from the light source 910 includes the horizontal componentalone, i.e., does not include the vertical component.

Reference character B designates waveforms (1031 and 1032) of thecontinuous wave light (see reference character B in FIG. 9) output fromthe polarization switch 920 to the specific-polarization modulator 930.The waveform 1031 and the waveform 1032 represent a horizontal componentand a vertical component of the continuous wave light output from thepolarization switch 920, respectively. As represented by the waveforms1031 and 1032, the continuous wave light output from the polarizationswitch 920 includes the signal component having a varying intensity thatis synchronized with the clock signal (see reference numeral 1010) andthe signal component having a varying intensity that is inverselysynchronized with the clock signal.

A waveform 1040 represents a waveform of DATA1 input to thespecific-polarization modulator 930. As represented by the waveform1040, DATA1 is input to the specific-polarization modulator 930 insynchronization with the clock signal (see reference numeral 1010). Awaveform 1050 represents a waveform of DATA2 input to thespecific-polarization modulator 942. As represented by the waveform1050, DATA2 is input to the specific-polarization modulator 942 ininverse-synchronization with the clock signal (see reference numeral1010).

Reference character C denotes waveforms (1061 and 1062) of thepolarization multiplexed light (see reference character C in FIG. 9)output from the specific-polarization modulator 942 to an externaldevice. The waveforms 1061 and 1062 respectively represent a horizontaland a vertical component of the polarization multiplexed light outputfrom the specific-polarization modulator 942.

As represented by the waveforms 1061 and 1062, the horizontal componentand the vertical component of the polarization multiplexed light outputfrom the specific-polarization modulator 940 to an external devicebecome signal components having varying intensities that are invertedwith respect to each other. Therefore, the polarization multiplexedlight output from the specific-polarization modulator 942 becomestime-interleaved polarization multiplexed light having a horizontalcomponent and a vertical component staggered with respect to each otherin terms of time (see FIG. 15).

As explained above, according to the polarization multiplexing andtransmitting apparatus 900 of the fourth embodiment, use of thepolarization switch 920 to convert the continuous wave light into thetwo signal components having varying intensities enables generation ofthe respective signal components having varying intensities using thesingle light source 910. When the continuous wave light is convertedinto the two signal components having varying intensities that areinverted with respect to each other, the respective signal componentshaving varying intensities can be temporally staggered with respect eachother without using the delay adjustment circuit (see reference numeral1750 in FIG. 17). Therefore, time-interleaved polarization multiplexingcan be executed while reducing apparatus size.

When the continuous wave light is converted into the two signalcomponents having varying intensities that are inverted with respect toeach other, the continuous wave light can be converted into the twosignal components having varying intensities without being attenuated.Therefore, the power consumed to obtain a necessary power of thepolarization multiplexed light can be reduced. For instance, thepolarization multiplexing and transmitting apparatus 900 can reducepower consumption by approximately half as compared with an example inwhich an optical divider attenuates the continuous wave light to beconverted into the signal components having varying intensities.

When a bit rate of the clock signal is variable, the polarizationmultiplexed light to be output can have a variable bit rate. Here, sincethe specific-polarization modulator 940 outputs the respective signalcomponents having varying intensities in a state such that these signalcomponents are always inverted with respect to each other, a temporalstaggering of the respective signal components having varyingintensities does not have to be adjusted according to a change in bitrate. Therefore, time-interleaved polarization multiplexing according toa variable bit rate can be performed with a simple configuration andcontrol.

FIG. 11 is a block diagram of a polarization multiplexing andtransmitting apparatus according to the fifth embodiment. As shown inFIG. 11, a polarization multiplexing and transmitting apparatus 1100according to the fifth embodiment includes a light source 1110, anoptical divider 1120, a specific-polarization modulator 1130, and aspecific-polarization modulator 1140. The light source 1110 generatesand outputs continuous wave light to the optical divider 1120 (referencecharacter A).

The optical divider 1120 receives the continuous wave light output fromthe light source 1110 and a clock signal. The optical divider 1120pulses the continuous wave light output from the light source 1110according to the clock signal, thereby providing a signal componenthaving a varying intensity that is synchronized with the clock signal.The optical divider 1120 outputs the pulsed signal component having avarying intensity to the specific-polarization modulator 1130.

Here, a polarization adjusting unit that adjusts a polarizationdirection of the signal component having a varying intensity output fromthe optical divider 1120 to a combined direction of a horizontaldirection and a vertical direction is provided. For example, thepolarization direction of the continuous wave light output from thelight source 1110 is set to a direction deviating from the horizontaldirection and the vertical direction at 45°. As a result, the signalcomponent having a varying intensity output from the optical divider1120 includes a light component whose polarization direction is ahorizontal direction and a light component whose polarization directionis a vertical direction.

The specific-polarization modulators 1130 and 1140 are equivalent to themodulator 130 depicted in FIG. 1. The specific-polarization modulator1130 receives the continuous wave light output from the optical divider1120 and DATA1. The specific-polarization modulator 1130 modulates onlya signal component having a varying intensity and a horizontalpolarization direction in the continuous wave light output from theoptical divider 1120, based on DATA1. The specific-polarizationmodulator 1130 modulates the signal component having a varying intensityin synchronization with the clock signal.

The specific-polarization modulator 1130 outputs the continuous wavelight including the modulated signal component having a varyingintensity and a horizontal polarization to the specific-polarizationmodulator 1140. The specific-polarization modulator 1140 receives thecontinuous wave light output from the specific-polarization modulator1130 and DATA2. The specific-polarization modulator 1140 modulates onlya signal component having a varying intensity and a verticalpolarization direction in the continuous wave light output from thespecific-polarization modulator 1130, based on DATA2. Thespecific-polarization modulator 1140, in synchronization with the clocksignal, modulates the signal component having a varying intensity.

The specific-polarization modulator 1140 outputs the continuous wavelight including the modulated signal component having a varyingintensity and a vertical polarization direction to an external device(reference character C). The specific-polarization modulator 1140includes a λ/2 wave plate 1141 and a specific-polarization modulator1142 like the specific-polarization modulator 940 depicted in FIG. 9.The λ/2 wave plate 1141 changes a polarization direction of thecontinuous wave light output from the specific-polarization modulator1130 by 90° and outputs this continuous wave light to thespecific-polarization modulator 1142. As a result, the light componenthaving a horizontal polarization direction and the light componenthaving a vertical polarization direction included in the continuous wavelight counterchange.

The specific-polarization modulator 1142, in synchronization with theclock signal, modulates only the signal component having a varyingintensity and a horizontal polarization direction in the continuous wavelight output from the λ/2 wave plate 1141. As a result, thespecific-polarization modulator 1130 can modulate the signal componenthaving a varying intensity and horizontal polarization direction in thecontinuous wave light output from the optical divider 1120, and thespecific-polarization modulator 1140 can modulate the signal componentwith a varying intensity and vertical polarization direction in thecontinuous wave light output from the optical divider 1120.

A polarization maintaining fiber that changes the polarization directionof the continuous wave light output from the specific-polarizationmodulator 1130 by 90° for output to the specific-polarization modulator1142 may be provided in place of the λ/2 wave plate 1141. Aspecific-polarization modulator that, in synchronization with aninverted clock signal, modulates only the signal component having avarying intensity and vertical polarization direction in the continuouswave light output from the specific-polarization modulator 1130 may beprovided in place of the λ/2 wave plate 1141 and thespecific-polarization modulator 1142.

DATA1 input to the specific-polarization modulator 1130 may be split,and the split DATA1 may be input to the specific-polarization modulator1140. The specific-polarization modulator 1140 compensates DATA2 usingDATA1, and executes modulation based on the compensated DATA2. DATA2input to the specific-polarization modulator 1140 may be split, and thesplit DATA2 may be input to the specific-polarization modulator 1130.The specific-polarization modulator 1130 compensates DATA1 using DATA2,and executes modulation based on the compensated DATA1.

As a result, even if the vertical component is modulated based on DATA1due to an error of the specific-polarization modulator 1130, thespecific-polarization modulator 1140 can compensate DATA2 using DATA1,to modulate the vertical component, thereby compensating the error.Further, even if the horizontal component is modulated based on DATA2due to an error of the specific-polarization modulator 1140, thespecific-polarization modulator 1130 can compensate DATA1 using DATA2 inadvance, to modulate the horizontal component, thereby compensating theerror.

FIG. 12 is a view of a time chart of the polarization multiplexing andtransmitting apparatus according to the fifth embodiment. In FIG. 12, anabscissa (t) represents a time common to light or electrical signals atrespective sections in the polarization multiplexing and transmittingapparatus 1100 according to the fifth embodiment. A waveform 1210represents the clock signal (CLK) input to the optical divider 1120.H-pol. of an ordinate indicates a horizontal component of a light.V-pol. indicates a vertical component of a light.

Reference character A denotes waveforms (1221 and 1222) of thecontinuous wave light (see reference character A in FIG. 11) output fromthe light source 1110 to the optical divider 1120. The waveform 1221represents a horizontal component of the continuous wave light outputfrom the light source 1110. The waveform 1222 represents a verticalcomponent of the continuous wave light output from the light source1110. As represented by the waveform 1221 and the waveform 1222, thecontinuous wave light output from the light source 1110 includes thehorizontal component and the vertical component at substantially thesame percentages.

Reference character B designates waveforms (1231 and 1232) of thecontinuous wave light (see reference character B in FIG. 11) output fromthe optical divider 1120 to the specific-polarization modulator 1130.The waveforms 1231 and 1232 represent a horizontal component and avertical component of the continuous wave light output from the opticaldivider 1120. As represented by the waveform 1231 and the waveform 1232,the continuous wave light output from the optical divider 1120 becomesthe signal component having a varying intensity that is synchronizedwith the clock signal (see reference numeral 1210).

A waveform 1240 represents a waveform of DATA1 input to thespecific-polarization modulator 1130. A waveform 1250 represents awaveform of DATA2 input to the specific-polarization modulator 1142. Asrepresented by the waveforms 1240 and 1250, both DATA1 input to thespecific-polarization modulator 1130 and DATA2 input to thespecific-polarization modulator 1142 are input in synchronization withthe clock signal (see reference numeral 1210).

Reference character C denotes waveforms (1261 and 1262) of thepolarization multiplexed light (see reference character C in FIG. 11)output from the specific-polarization modulator 1142 to the externaldevice. The waveform 1261 represents a horizontal component of thepolarization multiplexed light output from the specific-polarizationmodulator 1142. The waveform 1262 represents a vertical component of thepolarization multiplexed light output from the specific-polarizationmodulator 1142.

As represented by the waveforms 1261 and 1262, the horizontal componentand the vertical component of the polarization multiplexed light outputfrom the specific-polarization modulator 1142 to external unit becomethe signal components having varying intensities that are synchronizedwith each other. Therefore, the polarization multiplexed light outputfrom the specific-polarization modulator 1142 becomes time-alignedpolarization multiplexed light having a horizontal component and avertical component that are synchronized-with each other (see FIG. 14).

As explained above, according to the polarization multiplexing andtransmitting apparatus 1100 of the fifth embodiment, use of the opticaldivider 1120 to convert the continuous wave light into the two signalcomponents having varying intensities enables generation of therespective signal components having varying intensities by using thesingle light source 1110. Therefore, time-aligned polarizationmultiplexing can be executed while reducing apparatus size.

FIG. 13 is a block diagram of a polarization multiplexing andtransmitting apparatus according to the sixth embodiment. In FIG. 13,like reference numerals denote structures identical to those in thepolarization multiplexing and transmitting apparatus 400 depicted inFIG. 4, thereby omitting an explanation thereof. In FIG. 13, the clockoscillator 410 and the inverting circuit 411 depicted in FIG. 4 areomitted.

As shown in FIG. 13, a polarization multiplexing and transmittingapparatus 1300 according to the sixth embodiment includes an opticalreceiver 1310 and a controller 1320 in addition to the structures in thepolarization multiplexing and transmitting apparatus 400 depicted inFIG. 4. Polarization maintaining fibers 451 and 452 output respectivesignal components having varying intensities output from a phasemodulator 441 and a phase modulator 442 to a polarization beam combiner250 in a state in which all of a first to a fourth later-explained inputunique polarization state components of the polarization beam combiner250 are excited.

The polarization beam combiner 250 has two output units (output units1301 and 1302) that output coupled light to an external device. Thepolarization beam combiner 250 outputs a given polarization state (firstinput unique polarization state) component in coupled light to theoutput unit 1301 from the polarization maintaining fiber 451, and alsooutputs a second input unique polarization state component orthogonal tothe first input unique polarization state component to the output unit1302. The polarization beam combiner 250 outputs a given polarizationstate (third input unique polarization state) component in coupled lightfrom the polarization maintaining fiber 452 to the output unit 1301, andalso outputs a fourth input unique polarization state componentorthogonal to the third input unique polarization state component to theoutput unit 1302. At this time, the output unit 1301 outputs acombination of the first input unique polarization state component andthe third input unique polarization state component in polarizationstates orthogonal to each other, and the output unit 1302 outputs acombination of the second input unique polarization state component andthe fourth input unique polarization state component in polarizationstates orthogonal to each other as a monitor light.

The optical receiver 1310 is a monitoring unit that receives the monitorlight output from the output unit 1302 in the polarization beam combiner250 for conversion into an electrical signal. The optical receiver 1310outputs the converted electrical signal to the controller 1320. Theoptical receiver 1310 is, for example, a photo diode. The controller1320, based on the electrical signal output from the optical receiver1310, performs adjustment control over each section provided upstreamfrom the polarization beam combiner 250.

For example, as indicated by reference numeral 1330, the controller1320, based on the electrical signal output from the optical receiver1310, controls a bias supplied to a phase shifter 424 of an opticalswitch 420. As a result, auto bias control that adjusts an extinctionratio in a coupler 423 can be performed.

As indicated by a reference numeral 1340, the controller 1320 maycontrol respective signals supplied to the phase modulators 441 and 442to adjust modulation characteristics based on the electrical signaloutput from the optical receiver 1310. For example, the controller 1320performs auto bias control that controls a bias supplied to a λ/2 phaseshifter (not depicted) of the phase modulator 441 and the phasemodulator 442 to adjust an extinction ratio of the phase modulator 441and the phase modulator 442.

The polarization multiplexing and transmitting apparatus 1300 mayinclude an intensity adjuster 1351 that adjusts an intensity of a signalcomponent having a varying intensity output from the phase modulator 441to the polarization maintaining fiber 451 and an intensity adjuster 1352that adjusts an intensity of a signal component having a varyingintensity output from the phase modulator 442 to the polarizationmaintaining fiber 452. Each of the intensity adjusters 1351 and 1352 is,for example, an optical amplifier or an optical attenuator.

In this case, as indicated by reference numeral 1353, the controller1320 executes auto power control that, based on the electrical signaloutput from the optical receiver 1310, controls the intensity adjusters1351 and 1352 to adjust intensities of respective signal componentshaving varying intensities and output to the polarization maintainingfibers 451 and 452.

For example, as indicated by reference numeral 1360, the controller 1320may control torsion angles of the polarization maintaining fibers 451and 452 to adjust polarization directions of respective signalcomponents having varying intensities and output to the polarizationbeam combiner 250. This enables adjustment of a ratio of the signalcomponent having a varying intensity output from the output unit 1301and the monitor light output from the output unit 1302 as light to becoupled by the polarization beam combiner 250.

As explained above, according to the polarization multiplexing andtransmitting apparatus 1300 of the sixth embodiment, the effect of thepolarization multiplexing and transmitting apparatus 400 according tothe first embodiment can be demonstrated, and using the polarizationbeam combiner having two inputs and two outputs as the polarization beamcombiner 250 enables extraction of the monitor light without providing asplitter, e.g., an optical coupler. Thereby apparatus size can bereduced when adjustment of each section in the polarization multiplexingand transmitting apparatus 1300 is performed based on feedback control.

The example in which the sixth embodiment is applied to theconfiguration of the polarization multiplexing and transmittingapparatus 400 depicted in FIG. 4 is explained; however, the sixthembodiment can be applied to the respective polarization multiplexingand transmitting apparatuses according to the first to the thirdembodiments. Specifically, the polarization beam combiner having twoinputs and two outputs is used as the polarization beam combiner in eachof the polarization multiplexing and transmitting apparatuses accordingto the first to the third embodiments, and the optical receiver 1310,the controller 1320, and the intensity adjusters 1351 and 1352 areprovided.

As explained above, according to the polarization multiplexing andtransmitting apparatus of the present invention, time-interleavedpolarization multiplexing can be effected while reducing apparatus size.

The configuration in which the polarization multiplexing andtransmitting apparatus is used to perform time-interleaved polarizationmultiplexing is explained in each of the foregoing embodiments; howeverthe delay adjustment circuit (see reference numeral 1750 in FIG. 17)that delays one of respective signal components having varyingintensities may be further provided to synchronize the respective signalcomponents having varying intensities with each other, therebyperforming time-aligned polarization multiplexing (see reference numeral1400 in FIG. 14).

In this case, conversion of the continuous wave light into the twosignal components having varying intensities enables production of therespective signal components having varying intensities by using thesingle light source. Therefore, time-aligned polarization multiplexingcan be performed while reducing apparatus size. When the continuous wavelight is separated into the two signal components having varyingintensities that are inverted with respect to each other, the continuouswave light can be converted into the two signal components havingvarying intensities without being attenuated. Therefore, the powerconsumed to obtain a necessary power of the polarization multiplexedlight can be reduced.

According to the embodiments described above, a polarizationmultiplexing and transmitting apparatus realizes time-interleavedpolarization multiplexing while achieving a reduction in apparatus size.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

1. A polarization multiplexing and transmitting apparatus comprising: aconverting unit that converts light generated by a light source intosignal components having a varying intensity synchronized with a clocksignal input thereto and a varying intensity inversely synchronized withthe clock signal, respectively; a modulating unit that modulates thesignal components, respectively; and a polarization adjusting unit thatorthogonalizes polarization states of the signal components.
 2. Thepolarization multiplexing and transmitting apparatus according to claim1, wherein the modulating unit includes a first modulator that performsdata modulation with respect to one of the signal components based on afirst data signal and the clock signal, and a second modulator thatperforms data modulation with respect to the other of the signalcomponents based on a second data signal and an inverted clock signalthat is the clock signal inverted.
 3. The polarization modulatingapparatus according to claim 2, wherein the first modulator and thesecond modulator temporally stagger the signal components that are inthe polarization states orthogonalized by the polarization adjustingunit, by an amount corresponding to a half of a pulse repetition periodthereof and output the signal components.
 4. The polarizationmultiplexing and transmitting apparatus according to claim 1, furthercomprising a multiplexing unit that performs polarization multiplexingwith respect to the signal components respectively modulated by themodulating unit and in the polarization states orthogonalized by thepolarization adjusting unit, wherein the converting unit is an opticalswitch that distributes the light to two paths at a rate according tothe clock signal to convert the light into the signal components.
 5. Thepolarization multiplexing and transmitting apparatus according to claim4, wherein the optical switch includes a splitter that splits the light,parallel waveguides that transmit the light split by the splitter, aphase shifter that, according to the clock signal, changes a phase ofthe light passing through one of the parallel waveguides, and a couplingunit that couples the light respectively passing through the parallelwaveguides and distributes the light coupled thereby to the two paths ata rate according to interference conditions.
 6. The polarizationmultiplexing and transmitting apparatus according to claim 1, whereinthe converting unit and the polarization adjusting unit include apolarization switch that, according to the clock signal, changes apolarization state of the light to a first polarization state and asecond polarization state orthogonal to the first polarization state,and a polarization beam splitter that separates the light having thepolarization state changed by the polarization switch into the signalcomponents that are in the first polarization state and the secondpolarization state, respectively, and the polarization multiplexing andtransmitting apparatus further comprises a multiplexing unit thatperforms polarization multiplexing with respect to the signal componentsseparated by the polarization beam splitter and modulated by themodulating unit.
 7. The polarization multiplexing and transmittingapparatus according to claim 1, further comprising a multiplexing unitthat performs polarization multiplexing with respect to the signalcomponents that are respectively modulated by the modulating unit and inthe polarization states orthogonalized by the polarization adjustingunit, wherein the converting unit includes a splitter that splits thelight into a first light component and a second light component foroutput to two paths, and a pulsing unit that pulses the first lightcomponent in synchronization with the clock signal and pulses the secondlight component in inverse-synchronization with the clock signal.
 8. Thepolarization multiplexing and transmitting apparatus according to claim1, wherein the converting unit and the polarization adjusting unit are apolarization switch that, according to the clock signal, changes apolarization state of the light to a first polarization state and asecond polarization state orthogonal to the first polarization state,and the polarization multiplexing and transmitting apparatus furthercomprises: a first modulating unit that modulates a signal component inthe first polarization state among the signal components; and a secondmodulating unit that is connected with the first modulating unit inseries and modulates a signal component in the second polarization stateamong the signal components.
 9. The polarization multiplexing andtransmitting apparatus according to claim 8, wherein the secondmodulating unit includes a birefringent element that changes thepolarization state of the light modulated by the first modulating unitto the second polarization state, the light being polarizationmultiplexed light, and a modulator that modulates the signal componentin the first polarization state included in the polarization multiplexedlight that has passed through the birefringent element.
 10. Thepolarization multiplexing and transmitting apparatus according to claim8, wherein the first modulating unit compensates a first data signalusing a second data signal and modulates based on the first data signalcompensated thereby, and the second modulating unit compensates thesecond data signal using the first data signal and modulates based onthe second data signal compensated thereby.
 11. The polarizationmultiplexing and transmitting apparatus according to claim 4 wherein themultiplexing unit is a polarization beam combiner that couples thesignal components that are modulated by the modulating unit and have thepolarization states orthogonalized by the polarization adjusting unit.12. The polarization multiplexing and transmitting apparatus accordingto claim 6, wherein the multiplexing unit is a polarization beamcombiner that couples the signal components that are modulated by themodulating unit and have the polarization states orthogonalized by thepolarization adjusting unit.
 13. The polarization multiplexing andtransmitting apparatus according to claim 7, wherein the multiplexingunit is a polarization beam combiner that couples the signal componentsthat are modulated by the modulating unit and have the polarizationstates orthogonalized by the polarization adjusting unit.
 14. Thepolarization multiplexing and transmitting apparatus according to claim11, wherein the polarization beam combiner includes a first input unit,a second input unit, a first output unit, and a second output unit, thefirst input unit and the second input unit being complementary to thefirst output unit and the second output unit, the first input unitguides a first unique polarization state component to the first outputunit and a second unique polarization state component to the secondoutput unit, the second input unit guides a third unique polarizationstate component to the first output unit and a fourth uniquepolarization state component to the second output unit, the polarizationadjusting unit adjusts the polarization states of the signal componentssuch that the first to the fourth unique polarization states becomeexcited with an amplitude that is not 0, and the polarizationmultiplexing and transmitting apparatus further includes a monitoringunit that monitors a monitoring light output from the second outputunit.
 15. The polarization multiplexing and transmitting apparatusaccording to claim 12, wherein the polarization beam combiner includes afirst input unit, a second input unit, a first output unit, and a secondoutput unit, the first input unit and the second input unit beingcomplementary to the first output unit and the second output unit, thefirst input unit guides a first unique polarization state component tothe first output unit and a second unique polarization state componentto the second output unit, the second input unit guides a third uniquepolarization state component to the first output unit and a fourthunique polarization state component to the second output unit, thepolarization adjusting unit adjusts the polarization states of thesignal components such that the first to the fourth unique polarizationstates become excited with an amplitude that is not 0, and thepolarization multiplexing and transmitting apparatus further includes amonitoring unit that monitors a monitoring light output from the secondoutput unit.
 16. The polarization multiplexing and transmittingapparatus according to claim 13, wherein the polarization beam combinerincludes a first input unit, a second input unit, a first output unit,and a second output unit, the first input unit and the second input unitbeing complementary to the first output unit and the second output unit,the first input unit guides a first unique polarization state componentto the first output unit and a second unique polarization statecomponent to the second output unit, the second input unit guides athird unique polarization state component to the first output unit and afourth unique polarization state component to the second output unit,the polarization adjusting unit adjusts the polarization states of thesignal components such that the first to the fourth unique polarizationstates become excited with an amplitude that is not 0, and thepolarization multiplexing and transmitting apparatus further includes amonitoring unit that monitors a monitoring light output from the secondoutput unit.
 17. The polarization multiplexing and transmittingapparatus according to claim 14, further comprising a control unit thatcontrols setting adjustments upstream from the multiplexing unit basedon a monitoring result of the monitoring unit.
 18. The polarizationmultiplexing and transmitting apparatus according to claim 15, furthercomprising a control unit that controls setting adjustments upstreamfrom the multiplexing unit based on a monitoring result of themonitoring unit.
 19. The polarization multiplexing and transmittingapparatus according to claim 16, further comprising a control unit thatcontrols setting adjustments upstream from the multiplexing unit basedon a monitoring result of the monitoring unit.
 20. The polarizationmultiplexing and transmitting apparatus according to claim 1, furthercomprising a delay adjusting unit that delays one of the signalcomponents with respect to the other of the signal components andsynchronizes the signal components with each other.
 21. A polarizationmultiplexing and transmitting apparatus comprising: an optical switchthat distributes light generated by a light source to two paths at arate according to a clock signal input thereto to be convert the lightinto signal components having varying intensities that are inverted withrespect to each other; a modulating unit that modulates the signalcomponents; a polarization adjusting unit that orthogonalizespolarization states of the signal components; and a multiplexing unitthat performs polarization multiplexing with respect to the signalcomponents modulated by the modulating unit and having the polarizationstates orthogonalized by the polarization adjusting unit.
 22. Apolarization multiplexing and transmitting apparatus comprising: apolarization switch that, according to an input clock signal, changes apolarization state of light generated by a light source to a firstpolarization state and a second polarization state orthogonal to thefirst polarization state; a polarization beam splitter that separatesthe light having the polarization state changed by the polarizationswitch, into signal components that are respectively in the firstpolarization state and the second polarization state, and respectivelyhave a varying intensity; a modulating unit that modulates the signalcomponents; and a multiplexing unit that performs polarizationmultiplexing with respect to the signal components modulated by themodulating unit.
 23. A polarization multiplexing and transmittingapparatus comprising: a splitter that splits light generated by a lightinto light components for output to two paths; a pulsing unit thatpulses one of the light components in synchronization with a clocksignal input thereto and pulses the other of the light components ininverse-synchronization with the clock signal; a modulating unit thatmodulates signal components having varying intensities pulsed by thepulsing unit; a polarization adjusting unit that orthogonalizespolarization states of the signal components with respect to each other;and a multiplexing unit that performs polarization multiplexing withrespect to the signal components modulated by the modulating unit andhaving the polarization states orthogonalized by the polarizationadjusting unit.
 24. A polarization multiplexing and transmittingapparatus comprising: a polarization switch that, according to a clocksignal input thereto, changes a polarization state of light generated bya light source to a first polarization state and a second polarizationstate orthogonal to the first polarization state; a first modulatingunit that modulates a signal component that has a varying intensity, isin the first polarization state, and is in the light having thepolarization state changed by the polarization switch; and a secondmodulating unit that is connected with the first modulating unit inseries and modulates a signal component that has a varying intensity, isin the second polarization state, and is in the light having thepolarization state changed by the polarization switch.
 25. Apolarization multiplexing and transmitting apparatus comprising: apulsing unit that, in synchronization with a clock signal input thereto,pulses light generated by a light source; a polarization adjusting unitthat adjusts a polarization state of a signal component having a varyingintensity and pulsed by the pulsing unit, to a combined state of a firstpolarization state and a second polarization state orthogonal to thefirst polarization state; a first modulating unit that modulates asignal component that has a varying intensity, is in the firstpolarization state, and is in the light that is pulsed by the pulsingunit and has the polarization state adjusted by the polarizationadjusting unit; and a second modulating unit that is connected with thefirst modulating unit in series and modulates a signal component thathas a varying intensity, is in the second polarization state, and is inthe light that is pulsed by the pulsing unit and has the polarizationstate adjusted by the polarization adjusting unit.